A dispenser has generally the structure as shown in FIGS. 1A, 1B, 2A and 2B, in which a syringe 2 comprising a cylinder 20 and a piston 21 slidably engaging with the cylinder 20 is assembled to and supported on a bottom side of a dispensing head a (an external contour of which is shown with a chain line in FIGS. 1A-1B, but an internal structure of which is not shown in the figures) supported on a machine body 1 (a portion of which is shown with a chain line, but most portions of which are not shown) and capable of moving in three directions, namely in the right-to-left direction, in the front-to-back direction, and in the vertical direction) in the state where a piston 21 with the cylinder 20 fixed thereto can freely move up and down. When an upper edge side of the piston 21 of the syringe 2 is driven by a stepping motor M provided in a vessel for the dispensing head a and a screw shaft 32 rotates in the regular (forward) direction or in the reverse direction, an operating member 33 and an elevating mechanism 3 move up and down together. When the elevating mechanism 3 is lowered, the piston 21 moves downward as shown in FIGS. 1A and 2A to discharge a sample or a reagent, and when the elevating mechanism 3 is lifted, the piston 21 moves upward to suction the sample or the reagent. Furthermore, a chip b molded separately is dismountably engaged with a nozzle section 20a provided at a lower edge side of the syringe 2 as shown in FIGS. 2A and 2B. With this configuration, when a sample or a reagent is to be suctioned or discharged, the dispensing head a is moved to position the syringe 2 above a vessel c storing a sample, a reagent or the like therein. Then, when the dispensing head a is lowered, a tip of the chip b set on the nozzle section 20a of the syringe 2 is inserted into a liquid stored in the vessel c. In this state, when the elevating mechanism 3 is lifted to lift the piston 21 for the suction operation, the liquid is sucked into the chip b as shown in FIG. 2. After the sample or reagent liquid is sucked into the chip b, the dispensing head a is moved to a position above the vessel, such as a titration plate into which the liquid is to be poured, and the piston 21 is lowered to push out and dispense the liquid which had been sucked into the chip 2 to the vessel.
The prior art-based dispenser as described above has the problem as described below.
In the state where the chip b mounted on the nozzle section 20a of the syringe 2 has been inserted into a sample or a reagent stored in the vessel c as shown in the FIG. 2A, when the stepping motor M operates to drive the elevating mechanism 3 for lifting the piston 21, the liquid, having the volume calculated by multiplying a lifting stroke of the piston 21 by a cross section of the piston 21, is sucked into the chip b. In this sucking operation carried out when the piston 21 is elevated, however, also any air residing in a range from a lower edge portion of the inner space of the cylinder 20 to an inner space of the chip b is sucked together with the liquid of the sample or reagent into a space generated when the piston 21 is elevated, and an air pool is formed between a surface G of the liquid sucked into the chip b and a lower edge portion of the piston 21. Thus, an air layer is generated in the range from the lower edge portion of the piston 21 to the liquid surface G of the liquid having been sucked into the chip b.
The air layer H existing between the lower edge surface of the piston 21 and a surface of the liquid of a sample or a reagent sucked into the chip b functions as a cushion, because the liquid surface G is pressed down via the air layer H when the piston 21 is lowered for dispensing the liquid in the chip b. The cushioning action of the air layer H does not give any negative influence to the dispensing operation, because the air layer H compresses or expands only a little, and movement of the piston 21 is directly delivered to the surface G of the liquid within the chip b and a lowering stroke of the piston 21 accurately defines a volume of the liquid to be dispensed when the piston 21 is lowered for pushing out and dispensing a liquid of a sample or a reagent.
On the other hand, in the dispensing operation with a dispenser, an operation for dispensing a liquid such as a sample or a reagent into each of testing holes provided in an array or arrays on a titration plate is carried successively. Therefore, when a number of samples are to be examined all at once like in, for instance, an inspection facility where a number of samples are gathered, a long time is required for the dispensing work, which disadvantageously lowers the work efficiency.
If a lowering speed of the piston 21 is increased for improving the work efficiency, the air layer is compressed to cause the cushioning effect, and it becomes disadvantageously impossible to decide an accurate dispensing rate corresponding to the lowering stroke. This negative effect becomes more remarkable when a minute quantity of a sample or a reagent is to be dispensed successively and quickly.